In recent electronic devices and the like, optical wirings are employed in addition to electric wirings to cope with increase in information transmission amount. With a trend toward size reduction of the electronic devices and the like, there is a demand for a wiring board which has a smaller size and a higher integration density so as to be mounted in a limited space. For example, an opto-electric hybrid board as shown in FIG. 7A is proposed as such a wiring board, in which an opto-electric module portion E including an electric wiring 13 of an electrically conductive pattern and an optical element 10 mounted on pads 13a is provided on each (or one of opposite end portions of a front surface of an insulation layer 12 such as of a polyimide, and an optical waveguide W including an under-cladding layer 20, a core 21 and an over-cladding layer 22 is provided on a back surface of the insulation layer 12 (see, for example, PTL 1).
In the opto-electric hybrid board, an optical signal transmitted through the core 21 of the optical waveguide W as indicated by a one-dot-and-dash line P in FIG. 7A is converted into an electric signal by the optical element 10 of the opto-electric module portion E for electrical control. Further, an electric signal transmitted through the electric wiring 13 is converted into an optical signal by the optical element 10. The optical signal is transmitted through the optical waveguide W to another opto-electric module portion (not shown) provided on an opposite side, and taken out as an electric signal again.
In the opto-electric hybrid board, the insulation layer (such as of the polyimide) 12 contacts the optical waveguide (such as of an epoxy resin) W. Therefore, the optical waveguide W is liable to be stressed or slightly warped due to a difference in linear expansion coefficient between the insulation layer 12 and the optical waveguide W by an ambient temperature. Problematically, this increases the light transmission loss of the optical waveguide W. When the optical element for the optical-to-electric signal conversion and the electric-to-optical signal conversion and an IC for driving the optical element are to be mounted on the opto-electric module portion E, a mount surface of the opto-electric module portion E is liable to be unstable without provision of a reinforcement layer. Therefore, it will be impossible to properly mount the optical element and the IC on the opto-electric module portion E or, if possible, the opto-electric module portion E will fail to establish a sufficiently reliable connection.
To cope with this, it is proposed to provide a metal reinforcement layer 11 such as a stainless steel layer on the back surface of the insulation layer 12 to impart the opto-electric module portion E with a certain level of rigidity, whereby the stress and the slight warpage of the optical waveguide W are prevented to suppress the increase in light transmission loss. Without provision of such a metal reinforcement layer 11 in an interconnection portion of the opto-electric hybrid board other than the opto-electric module portion E, it is possible to ensure the flexibility of the optical waveguide W, so that the opto-electric hybrid board can be mounted in a smaller space to establish optical and electrical connections in a complicated positional relationship.